1. Field of the Invention
This invention relates in general to air exchange systems and, in particular, to an improved energy recovery ventilator (ERV), a cross flow plate core associated therewith and a method of conditioning air for a building.
2. Description of the Prior Art
Modern construction of commercial or residential buildings focuses on energy efficient building techniques which create airtight structures to minimize, in particular, heat loss in the winter and to reduce air conditioner use in the summer. However, unless properly ventilated, an airtight home can seal in indoor air pollutants and/or moisture. Contaminants such as formaldehyde, volatile organic compounds, and radon can accumulate in poorly ventilated homes, causing health problems. Excess moisture in a home can generate high humidity levels which can lead to mold growth and structural damage. Heating, ventilation and air conditioning (HVAC) systems in modern home construction include a furnace or similar device for heating in the winter along with an air conditioning device for cooling in the winter. Preferably, an integrated ventilation system is also included in new home construction to reduce the load on the heating and cooling components of the HVAC system.
With respect to the ventilation system, organizations such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) suggest that an indoor living area should be ventilated at a rate of 0.35 air changes per hour or 15 cubic feet per person per minute, whichever is greater. There are three basic ventilation strategies to exchange indoor air with outdoor air:                (a) Natural ventilation which provides uncontrolled air movement into a home through cracks, small holes, and vents, such as windows and doors. In a modern airtight home natural ventilation is limited;        (b) Whole-structure ventilation which provides controlled air movement using one or more fans and duct systems. Such systems are particularly useful in airtight homes; and        (c) Spot ventilation which provides controlled air movement using localized exhaust fans to quickly remove pollutants and moisture at their source.        
Whole-structure ventilation systems provide controlled, uniform ventilation throughout a building or home. These systems use one or more fans and duct systems to exhaust stale air and/or supply fresh air to the house. There are four types of systems:                (a) Exhaust ventilation systems which force inside air out of a home;        (b) Supply ventilation systems which force outside air into the home;        (c) Balanced ventilation systems which force equal amounts quantities of air into and out of the home; and        (d) Energy recovery ventilation systems which transfer heat and/or moisture from incoming or outgoing air to minimize energy loss        
There are two types of ERV systems: heat-recovery ventilators (HRV) and energy-recovery (or enthalpy-recovery) ventilators (ERV). Both types include a heat exchanger, one or more fans to push air through the heat exchanger, and some controls. The main difference between a heat-recovery and an energy-recovery ventilator is the way the exchanger works with respect to the transfer of either sensible heat or total latent heat energy. A heat-recovery ventilator is a sensible device which will result in a change of the air flow temperature only, while an energy-recovery ventilator is a total enthalpic device which results in a change of both the temperature and humidity within the air flow. In winter, an energy-recovery ventilator transfers some of the moisture from the exhaust air to the usually less humid incoming winter air, so that the humidity of the building air stays more constant. This also keeps the heat exchanger core warmer, minimizing problems with freezing. In the summer, an energy-recovery ventilator also helps to control humidity in the house by transferring some of the water vapor in the incoming air to the theoretically drier air that's leaving the house. In conjunction with an air conditioner, an energy-recovery ventilator generally offers better humidity control than a heat-recovery system. Most energy recovery ventilation systems can recover about 70%-80% of the energy in the exiting air and deliver that energy to the incoming air. Generally speaking, energy-recovery ventilators are most cost effective in climates with extreme winters or summers, and where fuel costs are high.
The key to an ERV system is the core which serves to exchange the heat/moisture between the incoming and outgoing air flows. There are two types of cores used in total enthaplic air exchangers: (a) Rotary wheel cores; and (b) Cross flow plate cores. With respect to (a), FIGS. 1A and 1B depicts a typical configuration of an air exchanger 6 with a rotary wheel core 8. Rotary wheel core 8 is filled with an air permeable material which provides a large surface area. The surface area is the medium for the sensible heat transfer. As rotary wheel core 8 rotates between the supply and exhaust air streams 10, 12 it picks up heat energy and releases it into the colder air stream. The driving force behind the heat exchange is the difference in temperatures between supply and exhaust air streams 10, 12, referred to as the thermal gradient. Typical media used in rotary wheel core 8 consist of polymer, aluminum, and synthetic fiber. The enthalpy exchange is accomplished through the use of desiccants. Desiccants transfer moisture through the process of absorption which is primarily driven by the difference in the partial pressure of vapor within the opposing air-streams. Typical desiccants consist of Silica Gel, and molecular sieves (to limit cross contamination). As shown in FIG. 1B which reflects summer conditions, the dry bulb (db) and wet bulb (wb) temperatures are both altered as air is passed through rotary wheel 8 e.g. both heat and moisture are removed from supply air stream 10.
With respect to (b), a cross-plate core arrangement 14 is depicted in FIG. 2A. Notably, air flows 16, 18 cross in the core and exchange heat/moisture. As will be understood in the art, the air flows do not physically interact, since the core consists of alternating layers of plates that are separated and sealed. FIG. 2B depicts a proprietary core structure offered by RenewAire highlighting how fresh and stale air flow streams 16, 18 are able to avoid coming in physical contact, yet are able to transfer heat/moisture through the configuration and type of membrane 20 which separates the air flows. The membrane 20 is typically treated paper or polyester fiber. As described in U.S. Pat. No. 6,413,298 issued Jul. 2, 2002 to Wnek et al. the membrane may consist of a sulfonated statistical copolymer. As will be appreciated, the construction of the core is essential to the efficient transfer of heat/moisture.
Current plate type ERVs require a complex process to manufacturer since the flow passage is contained by the plane of the membrane above and below it. As highlighted in FIG. 2B, the simplest construction of plate type ERV core 14 consists of fresh and stale air flow streams 16, 18 being isolated by folding the membrane over a corrugate spacer. This is common for cross flow only cores where the flow passages are rectangular as shown in FIG. 2C. Accurate control of adhesives application is required to seal the seams where the edges of the folded membrane meet. In most cases the membrane can be expensive and not very durable. Additionally, the exposed membrane edges are vulnerable to damage from incidental contact. As also shown in FIG. 2C, in a traditional plate type ERV core 14, fresh and stale air flow streams 16, 18 flow straight through flow passages 21 with the exit and entrance conditions being the same. Heat/moisture is transferred through upper and lower membranes 20 from air flow stream 16 to air flow stream 18.
Counter current/co-current cross flow hybrids ERV cores (not shown) have more complex geometries. When the geometry is more complex than simple rectangles wrapping a spacer with membrane is very difficult and inefficient. The membrane must be cut into odd shapes and folded over at odd angles. Some manufacturers of this design extrude the plastic for the spacer elements onto the core as it is being built. This requires computer numerically controlled (CNC) equipment and the ability to extrude plastic at the same time. Further the next membrane layer must be glued onto the spacer. A complex glue application process is required to apply glue only to the spacer without contaminating the membrane.
From a manufacturing standpoint, the core must be easy to assemble to keep production and capitol investment cost low. From an operational standpoint, the materials used in the core must allow moisture exchange but not allow air, which is mainly nitrogen and oxygen, to exchange between air flows 16, 18. As well, the materials must be neither undamaged by contact with water nor allow particulate to be exchanged between air flows 16, 18. From a safety standpoint, the core must meet Underwriters Laboratories UL900 Class 2 for smoke and flame resistance. This standard determines combustibility and the amount of smoke generated for air filter units of both washable and throwaway types used for removal of dust and other airborne particles from air circulated mechanically in equipment and systems installed in accordance with the Standards for Installation of Air Conditioning and Ventilating Systems, NFPA 90A (Other Than Residence Type), and for Installation of Warm Air Heating and Air Conditioning Systems, NFPA 90B (Residence Type). With conventional spacers used to separate stale and fresh air flows, spacer rails normal to the flow direction at the outlet and inlet can melt together. However, this may not serve to completely block the flow of air and thereby limit the spread of fire.
Although the ERV cores presently available are adequate for their intended purpose, a cost-effective ERV core meeting the above criteria is needed.